1. Field of the Invention
This invention relates to thin film metal alloy magnetic recording media, and in particular to a thin film metal alloy disk for horizontal recording in which an alloy comprising cobalt and platinum forms the magnetic layer and in which a specific chromium alloy underlayer between the magnetic layer and the disk substrate improves the magnetic properties of the media.
2. Description of the Prior Art
Alloys of cobalt and platinum with various percentages of platinum concentration have been used as the magnetic material in thin film magnetic recording disks for horizontal recording. In such disks, the hexagonal close packed (HCP) crystalline structure of the cobalt-platinum (CoPt) alloy is formed on the substrate, or on an intermediate underlayer, so that the C-axis, i.e. the [002] axis, of the CoPt film is either in the plane of the film or has a component in the plane of the film.
The coercivity (H.sub.c) of CoPt films is dependent upon the composition of the platinum, with the maximum H.sub.c occurring at approximately 20 atomic percent (at. %) platinum. See J. A. Aboaf, et al., "Magnetic Properties and Structure of Co-Pt Thin Films", IEEE Trans on Magnetics, MAG-19, 1514 (1983), and M. Kitada, et al., "Magnetic Properties of Sputtered Co-Pt Thin Films", J. Appl. Phys. 54 (12), December 1983, pp. 7089-7094. The coercivity and other properties of cobalt-platinum films have been reported by Opfer, et al. in an article entitled "Thin-Film Memory Disc Development," Hewlett-Packard Journal, November 1985, pp. 4-10. A CoPt thin film magnetic recording medium with between 10-30 at. % Pt is described in U.S. Pat. No. 4,438,066, which is assigned to the same assignee as this application.
A thin film disk with a cobalt-platinum-chromium (CoPtCr) magnetic layer, wherein chromium (Cr) is added to improve the corrosion resistance of the magnetic layer, is described in Japanese patent application No. 198568, published May 22, 1984. The CoPtCr magnetic layer is deposited onto a nickel-phosphorus (NiP) film formed on a suitable substrate.
In order to improve the magnetic properties of the CoPt magnetic layer in certain types of disks, various types of underlayers may be formed between the substrate and the CoPt magnetic layer. The use of a Cr underlayer in a CoPt thin film disk is described in the above referenced article by Opfer, et al. and in European patent application No. 145157, published June 19, 1985 and assigned to the Hewlett-Packard Company. European patent application No. 140513, published May 8, 1985, and assigned to the same assignee as this application, describes various magnetic recording structures for horizontal recording in which a Co-based HCP magnetic layer is deposited onto a Co-based primarily body-centered-cubic (BCC) non-magnetic layer. One of the possible structures suggested in the European reference is a CoPt magnetic layer, with Pt between 1 and 35 at. %, formed on a XCo layer, where X is either Cr or vanadium (V) with X between 60 and 99 at. %. The European reference suggests that the in-plane orientation of the C-axis of the HCP magnetic layer is enhanced because of "pseudo lattice" matching of the (002) plane of the BCC underlayer with the (110) plane of the HCP magnetic layer.